The present invention refers to a screening device and a method of manufacture thereof that are particularly suitable for screen cylinders or screen plates which screen, filter, fractionate, or sort cellulose pulp suspensions (that is in the pulp and paper making industry), or other similar suspensions. The present invention thereby refers to screening devices, such as screen cylinders or bended or flat screening elements, for screening, filtrating, fractioning or sorting pulp suspensions in pulp and paper making industry or other similar suspensions. The present invention more particularly refers to screening devices of the type comprising a plurality of filter wires positioned at a small spacing parallel to each other, the plurality of filter wires forming a screening surface facing the pulp suspension to be screened and adjacent wires forming screening openings therebetween allowing an accept portion of the pulp suspension to flow therethrough. EP 0 316 570 suggests such a screening device in which the filter wires are fixed by welding, on the downstream side of the wires, to transversely extending slots in solid support elements, support rings or support bars. The screening devices may have various forms, e.g. be flat, bended, cylindrical or conical.
In known screening devices of this type the support elements, which form supports for the filter wires, are formed of solid bars, mainly rectangular or round in cross section and most typically positioned perpendicular to the filter wires.
The filter wires are generally fastened to the supporting bars by a welding process which gives rise to a number of disadvantages such as variability distortion, thermal stresses and burrs. The heat induced by the welding often cause distortion of the wires and changes in the screening opening width between adjacent wires. It is therefore difficult to get completely uniform screening openings, which means that the efficiency of the screen suffers. Today, when the desired width of screening openings may be as small as 0.1 mm, only minimal distortions are acceptable.
The thermal stresses and the burrs may also lead to failure in operation due to the loading on the screening device in the user""s process. Such loading may be either in the form of a constant load or a cyclic loading giving rise to failure by fatigue. Burrs may also catch fibers in the suspension, leading to gradual clogging of the screen or filter, or the formation of so called xe2x80x9cstringsxe2x80x9d which are very detrimental in the user""s process.
It has also been suggested, e.g. in U.S. Pat. Nos. 5,090,721 and 5,094,360, to connect filter wires of a certain xe2x80x9ckeyxe2x80x9d cross section into recesses, in the support bar, having the same xe2x80x9ckeyxe2x80x9d form. By means of bending the supporting bars into rings, the filter wires are clamped into position. This design, thereby, requires the manufacturing of a number of relatively complicated and therefore expensive recesses. Further, it can only be adapted to circular screens and screens, where the flow is from the inside to the outside of the circular screen.
In another known screening device the filter wires are fastened by looping them around support bars. Such a screen construction is strong, but the looping areas around the support bars is locally closing the openings and thereby reducing throughput of the screen. Also the looped areas tend to have cavities and uneven spots which are facing the suspension potentially causing fiber hang-up.
The above difficulties tend to result in poor quality of screening or mechanical weaknesses or to high manufacturing costs, it is therefore the object of the present invention to minimize the above mentioned drawbacks and provide an improved screening device and an improved method of manufacturing such device.
It is thereby also an object of the present invention to provide an easily manufactured and assembled screening device without thermally induced distortion of filter wires.
It is also an object of the present invention to provide an improved strong screening device with accurate and consistent screening openings, i.e. screening slots.
It is thereby further an object of the present invention to provide an improved method of manufacturing a screening device, so that uniform screening openings, i.e. good tolerances, are provided, whereby slots with very small widths may be manufactured.
It is further an object of the present invention to provide an improved screening device with minimum of burrs or other protruding elements causing accumulation of fibers on upstream side surfaces of support rods.
According to one aspect of the present invention a screening device is provided comprising the following components: A plurality of filter wires each comprising a first section and a second section opposite the first section, the second section having a base portion. At least one longitudinal one longitudinal support element having first and second side surfaces, a plurality of slots in the first side surface, and a cavity in the second side surface in open communication with the slots. And, the second sections received by the slots so that the base portions extend through the slots into the cavity, and the wires are supported by the at least one support element so that the wires are substantially parallel to each other and define screening openings therebetween adjacent the first sections thereof.
The wires are preferably fixed to the support element(s) by local deformation of the base portions in the cavities, such as by effecting an increased material thickness portion of the base portion formed by reciprocation of a tool, by bending over a flap of the base portion, or by forming a weld between the base portion and the supporting element substantially within the cavity. Typically the second sections and base portions have a smaller cross-sectional area than the first sections. Alternatively the support element could be deformed within the cavity.
Typically the wires define screening slots having a slot width of less than 0.5 mm. The at least one support element may typically comprise a bar or a support ring; for example the at least one support element comprises a plurality of support rings, and the wires define a screen cylinder as the screening device. In the preferred embodiment the slots and wires are substantially transverse to the support rings, and the screen cylinder is positioned in a pressure screen so that the first sections of the wires and the first surfaces of the support rings impact slurry flowing through the screening slots.
The support element may be substantially U, L, or V-shaped in cross-section. Preferably the support element has a total height H, and the slots have a depth h2, wherein h2 equals (0.25-0.5) H; and the filter wires have a height h, and h2 equals (0.3-0.9) h.
According to another aspect of the present invention a method of manufacturing a screening device is provided comprising: (a) Providing a plurality of filter wires each having opposite first and second sections, and a base portion at the second section; and at least one longitudinal support element having first and second side surfaces, a plurality of slots in the first side surface, and a cavity in the second side surface in open communication with the slots. (b) Inserting the wires in the slots so that the base portions thereof extend into the cavity and the wires are substantially parallel to each other and define screening openings between the first sections thereof. And, (c) fixing the wires to the at least one support element substantially within the cavities.
Thereby a preferred screening device according to the present invention, comprising a plurality of filter wires supported by at least one longitudinal support element is provided, in which a plurality of supporting slots or recesses are made through the upstream side surface of the support element and the filter wires are fixed to the slots. The longitudinal direction of the supporting slots or recesses thereby form an angle, typically an angle of 90xc2x0, with the longitudinal axis of the support element and have a form adapted to receive the downstream section of the filter wires. The slots are typically cut perpendicularly into the support element, i.e. radially to the longitudinal axis of the support element, but may be cut at an angle between 10xc2x0 to 90xc2x0 into the support element, if the wires are to be supported in an inclined position. The filter wires are fixed to the slots or recesses by local deformation of the material in the downstream section of the filter wires or in the slot or recess limiting area of the support elements, after assembly of wires into the supporting slots in the support elements.
In a screening device, according to a preferred embodiment of the present invention, the at least one support element has on its upstream side supporting slots and on its downstream side a cavity delimited by side surfaces. The cavity may be formed by a variety of techniques including drawing, extrusion, rolling or machining. The plurality of supporting slots are preferably through openings reaching from the upstream side surface of the support element to the cavity. During assembly of the downstream section of a filter wire is inserted into the supporting slot in the support element the base portion thereof protruding through the slot into the cavity and preferably intersecting the cavity. The upstream side surface of the support element facing the suspension flow preferably has a rounded (convex) shape in order to reduce the flow resistance.
The slots, which may be formed e.g. by machining, stamping, spark erosion or laser, form an angle that intersects the axis of the support element. This angle is typically 90xc2x0 but could be within the range of 1xc2x0 to 90xc2x0. The spacing and the depth of supporting slots determine the position of the filter wires inserted therein and thereby also the width of the screening opening.
The filter wires are fixed to the support element by deforming the base portion of the downstream section of the wires, so that the deformation prevents the base portion from re-entering the slot and the wire from being pulled out. Filter wire material encapsulated within the support element cavity is preferably deformed by using mechanical force. The deformed material forms a mechanical joint, which has no burrs, but has good properties of fatigue resistance. The shape of the deformed material determines the ultimate performance of the joint in resisting forces generated by the filtration process. The form of the joint also determines the ultimate fatigue resistance of the jointed materials.
The shape of deformation may be determined by the tooling used to form the joints. The tool may e.g. have a flat, concave, convex, conical or domed form to cause material to flow in a direction determined to be optimal for the joint in question. Joints may be completed singly or in multiples in parallel filter wires to speed screening device production or ensure stability during processing. Other tooling may simultaneously be used to support adjacent supporting slots in the support elements to allow maximum force to be applied to the joints being formed, thus ensuring no distortion of adjacent support slots or filter wires occur. The support may be provided by the inserted filter wires being held in position by a clamping force.
Transverse slots in adjacent, preferably parallel, support elements should be in alignment to accept straight filter wire lengths. Filter wire material usually has to be straightened before assembly and connection to supporting slots.
According to another embodiment of the present invention, the filter wire is inserted into a supporting slot or recess, whereafter the support element material in the slot or recess area is locally (point wise or sectionally) deformed to press portions of the slot walls against the filter wire portion within the slot or recess. The deformation of the slot or recess is made at chosen locations to prevent the filter wire from being pulled out of the slot or recess. The slot or recess is preferably deformed by a mechanical force, such as pressing or stamping, directed onto the upstream side surface of the support element. The mechanical force is located so as to provide local deformation of the support element material around the slot or recess, without causing deformation or distortion of the whole support element and without causing distortion of the filter wire. The downstream section of the filter wire, inserted in the slot or recess, may be shaped in the slot or recess region to provide a space for deformed material and provide a re-entrant feature, so as to strengthen the joint. The deformation of the side surfaces is then adapted to lock the shaped wire in the slot or recess. If the slot is made as a through opening then both the base portion of the wire and the slot wall material may be deformed to provide a joint.
The support element and the filter wire are preferably supported during the mechanical deforming process to prevent undesired changes in the assembly.
The new method of manufacturing a screening device, according to a preferred embodiment of the present invention includes
forming in the upstream side surface of the at least one support element, by machining, cutting or another similar way, a plurality of supporting slots, which form an angle with the axis of the support element and are adapted to receive the downstream section of said filter wires,
inserting a filter wire of the plurality of filter wires in a supporting slot of the plurality of supporting slots, and
fixing the filter wire inserted in a supporting slot to the support element by locally deforming the material in the downstream section of the filter wire or in the slot limiting area of the support element.
In a screen cylinder according to the invention the support element is preferably a circular ring having a plurality of filter wires, parallel to the axis of the cylinder, fastened thereon. The filter wires may be fastened to the inner or outer periphery of the ring. Preferably there are at least two rings in each screen cylinder, but maybe more. The rings may simultaneously form supporting rings stabilizing the screen cylinder itself.
Preferably the plurality of supporting slots, made on the support element by machining or in any other suitable way, are mainly perpendicular to the longitudinal axis of the at least one support element, so that filter wires connected to the support element are perpendicular to said elements. It is, however, possible to provide inclined supporting slots on the support elements if desired, for inclined support.
The cross section of the filter wires preferably has a wider section facing the suspension to be screened and a narrower section protruding into the slots in the support element (support bar), for creating a relief channel between adjacent filter wires for the suspension to pass through. The width of the section facing the suspension is typically 2 to 8 mm, preferably 2,8 to 5 mm.
The support elements according to the invention may be made of a bar having a U- L- or V-shaped or other similarly shaped cross section. The bar thereby has a bent or an angled first, e.g. middle, portion onto which the filter wires are fastened and a second portion forming an additional support body. The convex or external side surface of the bent or angled first portion of the bar forms the upstream side surface facing the flow of suspension flowing through the screening device.
Typically a support element according to a preferred embodiment of the present invention is made of a partly solid support bar, the cross section of which is preferably slightly elongated, one end of the cross section being rounded or convex and the opposite end having a cavity formed therein. The support bar is disposed in the screening device, so that the rounded or convex side is arranged to face the flow coming through the screening openings formed between adjacent wires, for providing an optimal flow along the external surface of the support bar. The cavity in the support bar is thereby provided on the downstream side of the support element. The total height of the support bar is typically in the range of 10 to 25 mm, preferably 13 to 20 mm, and the width thereof in the range of 5 to 15 mm, preferably about 6 to 8 mm. The cavity protrudes typically about 5 to 15 mm, preferably 6 to 10 mm, into the downstream side of the support bar. The wall thickness of the support bar on the sides of the cavity may be 1 mm or more, typically about 1-3 mm.
Supporting slots are made into the convex or rounded upstream side of the support bar. The supporting slots typically have a depth h2 corresponding to 0.25 to 0.50 of the total height H of the support element. The supporting slots thereby may have a depth h2 0.3 to 0.9 of the height of the filter wires. The slots reach typically 1 to 3 mm deep into the cavity.
Wires having a height of about 5 to 15 mm, preferably about 7 to 12 mm, are supported by the support bars. The cross section of the wires has a funnel shaped wide upper (i.e. upstream) section, having a width decreasing in the downstream direction from preferably about 3 to 5 mm to about 1.5 to 3 mm in the upper ⅓ to xc2xd portion of the total height of the wire. The wire is inserted into the supporting slot, which preferably has a funnel shaped upper section corresponding to the form of the wire. The depth of the support slot and/or the funnel shaped upper ends of the slot and the wire determine the depth to which the wire may be inserted into the slot.
A base portion of the downstream end of the wire reaches according to a preferred embodiment of the present invention the cavity within the support bar. The wire is fixed to the support bar by providing a deformation to at least a portion of the wire portion reaching into the cavity, so that this deformation prevents the wire from being pulled out of the slot. The deformation may preferably be brought about by mechanically deforming, e.g. by stamping or swaging, at least a portion of the wire within the cavity. A deformation, according to the present invention, may alternatively be brought about by welding, soldering, gluing or in another similar non-releasable way, in which a fastening material is fixed to the downstream end of the wire, for attaching said wire to the inner walls of the cavity.
The support element may, according to another embodiment of the present invention, be made of a U-bar, having a material thickness of about 1-5 mm, preferably 1.5-2 mm. The middle portion of the U-bar has a bend with a radius of e.g. about 3-6 mm. A plurality of parallel supporting slots is made across the first middle portion of the bar, the supporting slots having a depth corresponding to xc2xc to xc2xd, advantageously ⅓ of the total height H of the U-bar. Preferably the supporting slots have a depth corresponding to ⅓ to ⅔ of the height h of a filter wire, whereby ⅔ to ⅓ of a filter wire inserted in a slot will still protrude above the supporting bar. The supporting slots may have a depth of 3-7 mm, e.g. 3,5 mm and the width of the upper portion of a supporting slot (in the longitudinal direction of the U-bar) may be about 1-3 mm, e.g. 1,5 mm.
The filter wire may, according to another embodiment of the present invention, be fastened to a supporting slot in a support bar, e.g. a U-bar or a partly solid bar having a cavity machined therein, by bending at least a portion of the downstream edge or base portion of the filter wire, protruding into the cavity of the support bar. Two preferably parallel notches may be provided perpendicular to the wire in the downstream edge of the wire, for providing an easily deformed or bendable flap. The notches are made long enough to enable the flap to be deformed or bent for locking the filter wire in the supporting slot and thereby fastening the wire to the bar.
The present invention is applicable in screen cylinders having inward or outward flow of suspension to be screened. In inward flow screens filter wires are connected to the external surface of supporting rings and in outward flow to the inner surface of the rings respectively.
The present invention provides a substantially improved screening device and method of manufacturing and assembling such device. The invention particularly provides an improved method of manufacturing a screening device, so that accurate and uniform screening slots, i.e. good tolerance, with very small widths may be manufactured. The new screening device provides a method of manufacturing a strong screening device with a minimum of burrs or other protruding elements causing accumulation of fibers.
The invention will be discussed in more detail in accordance with enclosed drawings in which